Plaque fluid and saliva samples from 4 irradiated monkeys (Macaca mulatta) and from groups of non-irradiated controls, all receiving a cariogenic diet, were analyzed on an individual basis for inorganic and organic constituents. Basic information on the composition of plaque fluid from animals with contrasting caries and on the effects of salivary gland irradiation is accumulating, giving some insights which may be relevant to caries in man.